Which one to pick, TH350 or TH400 ?

AuroraGirl

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If it comes to scrapping either trans id sell wichever you dont use. I say this because i had one HELL of a time finding a th350 as everybody scrapped them in recently passed years. Help another person out later by not scrapping it.
Good question for you, how about if a th350 case has a broken ear for the bellhousing, its one of the lower ones, passenger side if it is in a truck. since its cast aluminum is it just *****?
 

AuroraGirl

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Negative. Gotta be bolted to the frame
assuming the bed was even structurally sound to entertain the thought of bolting only to the bed, Id think it would be crazy to then make the weakest link in a hypothetically strong bed, frame setup is the 4 bolts holding the very expensive trailer to the truck.

Hard to ignore the fact that even with a robust bed floor(I would like to point out that old truck beds tended to be lighter gauge steel than later trucks) you dont want to leave the bed bolts as your murphys law lesson

however, if the OP Is going to be in the hills, wants to pull a heavy trailer, and wants to do it with his truck, he should also consider a brake upgrade if hes not already J55 rpo and making sure thats all to snuff. and the springpack should be brought up to F44 standards if not already
(or maybe it was C6P)

Yeah I think its c6p, c6p has larger brakes than regular 3/4 ton front and rear

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I dont remember if that was 84 or 82 but if the person I was showing this to a few weeks back was you, then i guess ti was for 83 lol. Notwithstanding, the idea is very simple. If you have anything short of a bonus/crew cab, you needed c6p to get the brakes, frame thickness, leafs, shocks, axle(maybe), engine cooling may be affected but youd be looking more at the power teams/gvw than the pure chassis alone since the powerplant is big on that.
looks like factory wheels would have wider too than c20 regular but not huge but just make sure if you have stock wheels youre matched on them(likely to be the same presumably Idk)
Even the c6p c20 is different from crew cab on the springs in back

Hills means braking, a Transmission can do a lot but that means you need cooler especially and whatever you dont stop with that means regular brakes. These trucks can stop on a dime but protect the investment and make sure youre at the best advantage you can be
 

AuroraGirl

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While the TH400 has some more mass to it, there's no real HP losses with running one. Plenty of dyno numbers on youtube to verify if you need feel like watching it. The argument comes up over the TH700 and the 4L80 when guys are LS swapping.
If you're towing, I like the TH400. It's not like the TH350 can't be tough. My buddy had an old Nova with one behind a 502. That car would pull a front wheel up when it launched, and that trans lived forever. However, the TH400 is beefier and will live longer when pulling. As Mark suggests, get a cooler and a temp gauge for whichever trans you go with.

Lastly, some friendly advice. If you're gonna tow a 5th wheel, use another truck. New ones are so much more stout than these old girls. If for no other reason that the better brakes on one, go find yourself a nice 3/4 ton silverado and tow safely.

Joe
I mean, it does use more HP on the efficient rotating mass wise, but its not anything you are gonna be apalled at how weak the truck is, esp if you already accounted for first gear being different. like your truck doesnt do just fine then no longer cuts the cake lol
 

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Good question for you, how about if a th350 case has a broken ear for the bellhousing, its one of the lower ones, passenger side if it is in a truck. since its cast aluminum is it just *****?
Depends on your comfort level... if it was a good trans and i know im not runnin high hp and it doesnt have the rest of the bell housing cracked id use it.... but thats me.
 

AuroraGirl

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Depends on your comfort level... if it was a good trans and i know im not runnin high hp and it doesnt have the rest of the bell housing cracked id use it.... but thats me.
I would think with the bell housing bottom cover with the torque rods if used would maybe cheat around the damage too lol.
 

Matt69olds

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I personally wouldn’t replace a good transmission just because it leaks. As far as heli coils are concerned, properly installed they are stronger than the case they are installed in. No problem there!!

Throughly clean the suspected leak area, use brake clean, power purple/simple green, pressure washer, whatever.

Go to your nearest auto parts store. Get a small bottle of A/C oil with the fluorescent dye, add about half the bottle to the fluid.

Drive the truck a few miles, then using a black light, look for the fluorescent dye. The dye will highlight the leakage point bright enough Stevie Wonder will be able to see it. I’m betting you will find either the dipstick tube or the seal around the kickdown cable is leaking.

If the dipstick tube is leaking, most likely it’s cracked or corroded where the O-ring sits in the groove. Replace it with a tube that takes the boot style seal, it’s a much better setup.

If it’s leaking from the kickdown cable, it’s possible the cable itself is cracked, or the wrong seal was installed.

There is no question a 400 is far more durable than a 350, but properly built a 350 will take some serious abuse.

Either way, if your going to work the truck hard, put the biggest cooler you can find on the truck, and install a deep transmission pan. Anything you can do to keep the trans cool will help it live.
 

77 K20

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I don't know, it will depend on what is available IF and when the house sells and I have some money to work with.
So much still up in the air right now.
I hope whatever I do get can safely be towed with the hitch bolted to the bed.
Drilling the frame and getting the right length spacers to fit between the frame and the bed underside will probably be a headache.
A lot of 5th wheel hitches were just bolted on to the bed- at least in the 80s. My dad bought a 18' 5th wheel in 1988 and the RV place did just that. He has towed it ever since (34 years). It's been towed all over the western US and Canada. He even towed my 21' 5th wheel a few times. But mine only weighs around 4,700 lbs fully loaded. However IF there was ever an accident while towing it might be pretty bad. :confused:

That being said to bolt the hitch rails to the frame isn't too hard. Mine is bolted to the frame with 4 L brackets. The upper part of the bracket goes thru the bed into the cross rails in the bed. Then you have multiple pre-drilled holes in the bottom part of the L bracket to bolt into your frame. I don't have a picture of this offhand...

etrailer has a lot of pics of this setup

Click above for some pictures at etrailer's website.
 

AuroraGirl

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A lot of 5th wheel hitches were just bolted on to the bed- at least in the 80s. My dad bought a 18' 5th wheel in 1988 and the RV place did just that. He has towed it ever since (34 years). It's been towed all over the western US and Canada. He even towed my 21' 5th wheel a few times. But mine only weighs around 4,700 lbs fully loaded. However IF there was ever an accident while towing it might be pretty bad. :confused:

That being said to bolt the hitch rails to the frame isn't too hard. Mine is bolted to the frame with 4 L brackets. The upper part of the bracket goes thru the bed into the cross rails in the bed. Then you have multiple pre-drilled holes in the bottom part of the L bracket to bolt into your frame. I don't have a picture of this offhand...

etrailer has a lot of pics of this setup

Click above for some pictures at etrailer's website.
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this is my first bed off my 1980 and the holes are hard to see but the thing had a gooseneck setup that also then went into the current bed too a long time ago. no longer around, they got rid of it after he was done with animals and they sold trailers. Which was in the 90s for some reason(they got rid of them in the 80s lol)
but the bed is also not flat at all, you can see its bent, wavy, one hole is clearly got punched through partially even, and then has a big washer holding it (bed holes) the hitch had square holes in spots. i assume for the rails/whatever.

I dont know if it got bolted to the frame too but can is different than should
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there is more but just snapped a quick shot of what big brother wants
 

bft305

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I should add that if you swap in the th400 you are probably looking at new driveshaft. Cause the length of a th350 and th400 are different. Might be able to get away with it but when I went from a np203 to np205 I had to get new driveshafts front and rear. Also I had a hell of a time with my th350 leaking ATF. I finally got some transmission RTV and did a skim coat then gasket and then a skim coat and pan. Sealed up great!
I also agree with JoeR Jr buy something else to tow a 5 wheel, you will be better off. As much as I love my square it is old and I don’t think I would trust it towing long distance. Just my 2 cents! Good luck with what ever you choose!
 

mtbadbob

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My experience is that a TH400 is far better for heavier duty job's. I've seen too many 4x4 'Burb's, Blazers & truck's go through TH350's when working them hard in 4x4 mode, or towing. Look at it this way...Chevy put 292's in a lot of their trucks, but the 350 outperformed them MOST of the time. Why go to McDonald's when you can eat a ribeye???
 

AuroraGirl

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My experience is that a TH400 is far better for heavier duty job's. I've seen too many 4x4 'Burb's, Blazers & truck's go through TH350's when working them hard in 4x4 mode, or towing. Look at it this way...Chevy put 292's in a lot of their trucks, but the 350 outperformed them MOST of the time. Why go to McDonald's when you can eat a ribeye???
the 292 has a simplicity and a stock speaking, cheap allure to it in the old days.

Plus less pathetic outputs stock would have been nice to see as well as better/more trans options
 

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